Within the past years heat engines have been developed which utilize the extraordinary properties of a nickel-titanium alloy called Nitinol to convert heat to mechanical energy. There exists the possibility that these small prototype solid-state engines may evolve into practical converters of waste heat.
The engines of the foregoing type are made possible by the discovery, in about 1960, of some remarkable properties possessed by nearly equi-atomic alloys of nickel and titanium. These alloys undergo an energetic crystalline phase change at ambient temperatures giving them a shape memory. The material if plastically deformed while cool will return to its undeformed shape when warmed, exerting great force as it does. This shape memory, energized by a solid-state phase change, has been utilized in several engine designs. Nitinol engines constructed prior to this invention are severely limited in thermodynamic efficiency; in these engines, cold Nitinol elements are heated by immersion in a hot fluid and these heated elements cooled in a heat sink at a much lower temperature. Such a cycle is irreversible because of the large amount of entropy generated, and consequently such engines cannot approach the efficiency of an ideal engine, or Carnot efficiency, ##EQU1## where T.sub.1 and T.sub.2 are the absolute temperatures of the hot and cold baths. An engine incorporating a heat regenerator overcomes this difficulty and can, in principle, be made to operate near Carnot efficiency.